Jim Klinger, Concrete Construction Specialist, The Voice Newsletter, January 2021

Hotline Question

“We have a project featuring a 6-inch thick concrete slab-on-ground over an 85,000 SF footprint of prepared subgrade. Reinforcing for the slab is plain welded-wire mesh (WWR 6x6). We installed the WWR using a lap splice of one square overlap, or 6 inches. The project inspector has rejected the laps, saying they must be one square plus 2 inches, or 8 inches total, minimum. The inspector is citing ACI 318-19, section 25.5.4, which covers plain wire WWR tension lap splices for structural slabs. While ACI 318-19 does call for the lap length cited by the inspector, we don’t agree with his interpretation. Neither the structural construction documents nor the WWR shop drawings indicated lap splices. How can we prove our installation is fit for purpose and acceptable?”

Hotline Answer

Since the WWR lap splice requirements are not available in the construction documents, it appears the inspector has mistakenly referred to Chapter 25 or the ACI 318 building code as a default to support his decision. We believe the as-placed WWR is likely to be accepted by the LDP with little or no rework required. Here’s why.

The non-structural slab-on-ground on this project was probably designed per ACI 360: Design of Slabs-on-Ground. By definition, such slabs are supported by the ground with the main purpose of transmitting all applied loads through bearing onto a properly prepared subgrade. Examples of non-structural slabs include industrial, commercial, and residential slabs intended to carry material loads, storage rack loads, and any static or moving loads from equipment and vehicles.

The WWR in the slab actually contributes very little to its load-carrying capacity. The slab thickness and proper preparation of the subgrade provides that. The WWR is used only for control of concrete slab shrinkage cracks.

Since the design requirements for non-structural slabs-on-ground are specifically excluded from the ACI 318-19 building code, section 1.4.8, the LDP typically uses design guides prepared by ACI, CRSI, WRI and so on. The LDP can also rely on past experience with similar slabs. Once the slab is designed, the LDP is required to show locations and details of all reinforcement, including all lap splices.

So, if the WWR is only needed for crack control in a non-structural slab-on-ground, what is the correct lap splice to use with plain wire mesh?

To answer this we checked ACI 439-5R-18: Guide for Specification, Manufacture and Construction Use of Welded Wire Reinforcement. In section 6.4, the lap splice indicated for plain wire WWR is one square, plus 2 inches. This is essentially the same ACI 318 lap splice for plain wire used in structural slabs.

Next, we found an old WRI design guide dated 1999 that contained the following note to slab designers:

“ACI 318 does not cover splices for slabs-on-ground. WRI believes and many engineers agree that the strength of the reinforcing in most slabs-on-ground is not utilized to the full yield strength. Most slabs-on-ground utilize less than 50% of the yield strength of the reinforcement. Therefore, splices need only to be sufficient to secure the sheets of WWR together. The minimum splice is 2” plus overhangs. If the engineer believes the full strength of the reinforcing will be required, then the splice equations in Chapter 12 apply”.

We contacted a WRI representative for more current guidance for splicing of plain wire WWR in non-structural slab applications.

Although the note to designers cited above no longer appears in the WRI design guide, the LDP is free to call out any splice length that can be proven sound to the local authority having jurisdiction e.g., building department. The trend appears to be for engineers to simplify the design process, avoid doing calculations, and use the conservative default splice lengths used in ACI 318 for structural slabs. In other words, it is totally up to the engineer to determine the lap splice required, if any. But you’re likely to see the one square plus 2 inches lap on your next project.

UPDATE: In a follow-up phone call, we learned that the ASCC member discussed the existing installation with the project LDP, who agreed the one-square lap was indeed fit for purpose and acceptable.